Significant advances have been made in recent years in the development of wireless communication systems for remote monitoring. In a typical system of that kind, a transponder is remotely located to monitor various parameters, such as temperature, status, and location, among others. Such wireless, remote monitors are attractive because they are reliable, inexpensive, and rugged.
The transponder includes a resonant circuit used for receiving signals (typically radio frequency ("RF") signals) from an interrogation unit, and for transmitting RF signals containing monitoring information back to the interrogation unit. Furthermore, energy from the signals received by the resonant circuit is stored and used to power the transponder, thus eliminating the need for power supplies in many applications. The stored power is also used to excite the resonant circuit for transmission of signals back to the interrogation unit.
To conserve energy, the resonant circuit is not continuously excited. Rather, it is initially excited and then periodically supplied with additional energy to maintain oscillation. The maintenance energy is needed because the resonant circuit has various losses, and other components of the transponder also cause a damping of the oscillation, thus resulting in decay of the oscillation. This re-excitation with maintenance energy pulses is referred to as plucking.
With existing transponders, the plucking function is initiated after a fixed number of oscillations. For example, the plucking function may be initiated after every eight oscillations (also known as ringings) of the resonant circuit. Unfortunately, the quality-factor of resonant circuits from transponder to transponder is not constant, and thus the frequency of the plucking function must be correspondingly changed for proper operation. For example, a plucking function that is performed every eight oscillations may be insufficient to maintain oscillations in a low quality-factor resonant circuit. Moreover, the quality-factor of individual resonant circuits may not be constant, due to nearby metal or circuits used to dampen other nearby resonators.